This invention relates to electrical protective apparatus and, more particularly, to means for visually indicating a malfunction in an electric power rectifier bridge whose separate legs are formed of parallel arrays of semiconductor power rectifier cells.
A power rectifier cell is physically characterized by a body of semiconductor material (usually silicon) sealed in an insulating enclosure or housing between a pair of main current-carrying metallic electrodes (anode and cathode, respectively). The semiconductor body comprises, for example, a thin, broad area disc-like wafer having adjoining layers of different conductivity types (P and N, respectively), whereby a PN (rectifying) junction is formed between the main electrodes. Such a device is often referred to as an uncontrolled rectifier or simply a diode. When electrically connected in series with a load impedance and an external power circuit, including a source of alternating voltage, a diode will ordinarily block the flow of load current so long as the potential of its anode is negative with respect to the cathode (i.e., reverse biased) but will freely conduct load current if the anode potential is relatively positive by at least a predetermined small threshold (i.e., forward biased).
The forward current and peak blocking voltage ratings of a diode are specified by the manufacturer. These ratings determine the maximum load current that the diode can conduct in the forward direction and the maximum applied voltage that it can safely withstand in the reverse direction. By way of example, power diodes having a maximum continuous RMS forward current rating of 250 amperes and a peak reverse voltage of 1,200 volts at an operating junction temperature of 160.degree. C are commercially available today.
Such diodes are used to rectify current in many high power applications. One such application is in the art of diesel electric locomotives where three-phase rectifier bridge are used to supply direct current (d-c) to traction motor loads from the three-phase alternating current (a-c) output of diesel engine-driven alternators. Each leg of such a rectifier bridge must be capable of carrying as much as 1,500 amperes in the forward direction, thereby requiring the paralleling of diodes. Thus the basic building block of the rectifier bridge is a parallel array of discrete diodes, all poled in agreement with one another. It is customary to provide either electric current limiting fuses or extra diodes in series with the respective paralleled diodes so that the equipment can remain operative if one of the diodes were to fail shorted, a condition wherein the failed diode looses its ability to block current when reverse biased. Where extra diodes are provided, all of the diodes are usually shunted by duplicate capacitors appropriately sized to promote the substantially equal division, among the seriesed diodes, of the reverse voltage that is periodically applied across each leg of the bridge. These capacitors also provide desirable snubber action.
When properly protected by current limiting fuses or extra series diodes, the occasional shorting of a weak or improperly installed diode will not cause a catastrophic short circuit between the a-c and d-c terminals of the affected leg of the rectifier bridge, and the remaining sound diodes usually perform adequately at least for a while. However, the safety factor has been reduced, and some of the remaining diodes will now experience higher electrical stress than normally intended and may soon fail, whereupon a serious line-to-line fault can occur with resulting damage to the rectifier bridge or to the a-c source. It is desirable, therefore, to routinely check for an insipient failure of the rectifier bridge before such damage actually occurs. Heretofore this has been done by continuity checks of all of the diodes at routing maintenance intervals, a time consuming and painstaking task at best, or by individually monitoring the diodes with relatively expensive indicating fuses or lamps.